This invention relates in general to splices for electrical conductors and, more specifically, to a splice configuration particularly useful with superconducting cable-in-conduit conductors of the sort used in superconducting magnetic energy storage systems.
Superconducting magnetic energy storage (SMES) systems are capable of storing large amounts of electricity in a DC magnetic field for indefinite periods. Power from a utility grid or other power source such as a wind turbine or solar plant can be stored until needed, then returned to the utility grid or any specific application at any time. Utility applications include load leveling, spinning reserve, transmission system stability and reliability and voltage/power factor correction.
A SMES system often includes a cable-in-conduit conductor, which includes a superconducting alloy, wound into a large diameter coil or solenoid. The conductor is cooled to a temperature at which it becomes superconducting. With present commercial superconducting materials, the conductor is cooled with liquid helium to -456.degree. F. (1.8.degree. K.). In the cable-in-conduit configuration the conductor includes spaced superconducting cables in an annular ring around a metal tube containing the helium coolant. Typical SMES coils frequently are very large in diameter, often having diameters of over 100 feet.
Because of the length of the coils, it is necessary to splice lengths of cable to form the continuous coil. In large coils, one or more splices must be provided for every coil turn. These splices must have electrical and physical properties that do not degrade the performance of the coil.
With the usual materials, such as copper or aluminum, used at ambient temperatures, two cables are generally simply mechanically fastened together or soldered together. Typical of the prior art techniques for joining two conductors together are the crimp rings disclosed by Bennett in U.S. Pat. No. 3,231,964, and soldering as described by Mcintosh et al in U.S. Pat. No. 3,517,150.
Superconducting cables, however, have physical characteristics making such simple splicing techniques either unworkable or difficult. Superconductors are materials, often metals or ceramics, that lose all resistance when cooled below a critical temperature. Many materials have superconducting capabilities, although most only superconduct at temperatures approaching 0.degree. K. The most practical for present use in superconducting magnets and the like are those that superconduct at or near the boiling temperature of liquid helium; typically, V.sub.3 Ga and NbTi alloys and the compound Nb.sub.3 Sn. The most common method of splicing such superconductors has been the lap splice, where the cable ends are overlapped and soldered together. Such soldered lap splices exhibit relatively high resistance which can lead to excessive local heating, to the point where the spliced superconductors are raised above the critical superconducting temperature and cease to superconduct.
A number of different methods have been developed in order to connect ends of superconductor cables without interposing a high resistance material, such as solder, between them. Where the cable has multiple strands, simply overlapping the strands of each cable and crimping them together has been proposed by wada et al. in U.S. Pat. No. 4,794,688. However, this is not effective with many superconductor cables and provides only a mechanical joint which may have insufficient strength for some applications.
Multi-filament cable ends have been joined by intertwining the superconductor filaments, heating to a diffusion temperature then crimping a sleeve over the connection as described by Smathers in U.S. Pat. No. 5,111,574. This is a complex process which may degrade the superconducting properties and would be difficult to consistently accomplish outside of a laboratory environment.
Jones, in U.S. Pat. No. 4,631,808 places two cable ends in parallel, crimps a sleeve of superconducting material over the ends, then embeds the entire assembly in a conductor. This method, however, is not suitable for a continuous cable to be wound into a coil or the like.
Thus, there is a continuing need for a simple but effective method of splicing ends of superconducting cables together to form a longer cable suitable for winding into magnet coils, in particular SMES coils and the like, without degrading the electrical and physical properties of the coil and the coil structure.